With the globalization of the elevator industry there has been a trend to standardize elevator systems worldwide. This trend is leaning toward the use of traction systems for smaller elevator applications (i.e., less floors). Previously, hydraulic elevator systems were commonly used in applications with over five landings. The trend anticipates that these applications will begin to utilize traction elevator systems. Such systems must be provided with emergency or back-up power systems that supply power not only to the controller, door operator, and valves, but also to the main drive system.
Many common back-up Universal Power Systems (UPS) use a high frequency waveform synthesis to create a near perfect three phase sine wave output waveform. This approach requires an expensive design. This approach can also cause problems for an elevator control system, as there will be high frequency noise and potentially a larger than expected number of zero crossings.
The present invention overcomes the disadvantage of the common back-up UPS by providing a stepped square wave output. Therefore, the invention provides the power required with a much simpler and less expensive design.
In addition, recent developments have lead to traction elevator systems replacing older technology, i.e., soft start systems, with new Variable Frequency Drive (VFD) technologies. VFD technology has two advantages. First, VFD technology allows a traction motor to be connected to the main power system with a low level of inrush current. Second, VFD technology allows a traction motor to run at a very low speed and at a very low power. Thus, while a typical traction motor might be a 60 hp three phase load, when running at a normal speed, the motor may only be a 2 hp load at its slowest speed. The reason for this low load is that a traction elevator system is counter weighted. The elevator's typical loading of passengers (i.e., the passenger weight) is exactly matched by the counter weight. This allows for optimal efficiency of the system. However, under most elevator conditions, an exact matching of the counter weight and the passenger load does not occur. Thus, a traction elevator will tend to drift up or down depending on this imbalance.
By continually monitoring the elevator load, the present invention is able to keep track of which direction the car would drift. When a power outage occurs, this information is available for use by the emergency back-up power system. Also, to handle the capacitive nature of the VFD, a three phase inductor system is placed between an inverter stepped square wave output stage and the VFD. This prevents the high dv/dt of the square wave from causing large load current levels.
Furthermore, unlike the hydraulic elevator systems, the traction elevator system requires that the back-up power system provide full power to the traction motor (e.g., >50 hp load at full motor speed). This requires that the back-up power system be capable of switching a high power load. In hydraulic applications, the back-up power system is not required to power the large hydraulic pump. The system is required to only power a valve that relieves the hydraulic pressure in the system thereby lowering the elevator. In the traction system, the requirement to handle high levels of normal power results in a system where the back-up power is fed in parallel to the normal control power system. This results in a need for a different system approach for sequencing the various systems so as to assure that both the back-up power and the normal control power sources are not connected to the traction elevator system simultaneously.